The tarnished former cop Ed McMellon – one of four officers to shoot and kill unarmed African immigrant Amadou Diallo in 1999 – became a full-fledged firefighter yesterday.

During a joyous graduation ceremony tinged with sorrow, 260 probationary firefighters received their diplomas at Brooklyn College.

But six seats were left vacant to honor the probationary firemen who rushed into the Twin Towers Sept. 11 to save lives and never came out. Purple-and-black bunting – reserved for fallen heroes – draped those empty chairs.

“It’s a day of mixed emotions,” said Fire Commissioner Thomas Von Essen. “They’ve been through so much more than [any] other class in the history of the department.”

At one point, Von Essen said, they’d considered canceling the ceremony.

“You can see how tough it is to sit there and be happy in front of families that just lost a son,” he said. But it was done “in the spirit of beginning a healing process.”

The class that graduated was the first to have “probies” assigned to firehouses as part of their training – and that’s why they rushed into action on Sept. 11. But Von Essen assured their parents that the new recruits who went into the World Trade Center were qualified. Those with the most experience – 40 years and 20 years – also died that day, he said.

Mayor Giuliani said, “You’ve had to face the worst . . . The loss of people you’ve worked with at a very, very early stage in your careers.”

“Have great pride,” he said. “Be very careful, be safe and keep showing us the example of love that’s inspiring the world.”